A gas sensor is known from the German Published Patent Application No. 199 41 051, for example, for use in analyzing the exhaust gas of internal combustion engines. The gas sensor regulates the air/fuel ratio of combustion mixtures in motor vehicle engines and is provided with a sensor element that combines a concentration cell (Nernst cell) with an electrochemical pump cell.
The concentration cell of the sensor element has a measuring electrode arranged in a measuring-gas region and a reference electrode located in a reference-gas region. The two electrodes are applied on a solid electrolyte member and electrically connected via the solid electrolyte member. The measuring-gas region in which the measuring electrode is located is connected to the exhaust gas outside of the sensor element via a diffusion barrier and a gas-access hole. The reference-gas region is in contact with a reference atmosphere via an opening situated on the side of the sensor element facing away from the measuring-gas region. Measuring-gas region and reference-gas region are arranged in the same plane of stratification of the sensor element configured as layer system and are separated by a gas-tight separation member. At different oxygen partial pressures in the measuring-gas region and the reference-gas region, a so-called Nernst voltage is generated between the measuring electrode and the reference electrode. When the oxygen partial pressure in the reference-gas chamber is constant, the oxygen partial pressure in the measuring-gas region may be determined from the Nernst voltage.
The pump cell of the sensor element includes an annular outer pump electrode arranged on an outer surface of the sensor element and exposed to the exhaust gas and an also annular inner pump electrode located in the measuring-gas region on the solid electrolyte member. The inner pump electrode may coincide with the measuring electrode of the Nernst cell or may be electrically connected to it. The outer pump electrode has a greater outer radius and a smaller inner radius than the inner pump electrode, so that the surface of the outer pump electrode is larger than the surface of the inner pump electrode. By means of supply lines, the electrodes are electrically connected to contact surfaces arranged on the side of the sensor element facing away from the electrodes. An insulation layer electrically insulates the supply lines of the electrodes, in particular the supply line of the outer pump electrode, from the solid electrolyte member.
When a pump voltage is applied between the outer pump electrode and the inner pump electrode, the pump cell pumps oxygen ions via the solid electrolyte member out of the measuring-gas region into the exhaust gas or vice versa from the exhaust gas into the measuring-gas region. The pump voltage is regulated by an external circuit elements in such a way that a Nernst voltage of approximately 450 mV is available between the electrodes of the Nernst cell, which corresponds to an oxygen partial pressure in the measuring-gas region of lambda=1 (stoichiometric air-fuel ratio). Accordingly, oxygen is pumped out of the measuring-gas region if lean exhaust gas is present (lambda>1), the pump current flowing in the pump cell being limited by the diffusion stream of the oxygen molecules flowing through the diffusion barrier into the measuring-gas region. In the case of rich exhaust gas (lambda<1), oxygen is pumped into the measuring-gas region, and the pump current flowing in the pump cell is limited by the diffusion stream of the gas molecules that flow through the diffusion barrier and consume oxygen in the measuring-gas region (the oxygen pumped into the measuring-gas region reacts there with the oxygen-consuming gas molecules). In lean exhaust gas, the diffusion stream is proportional to the oxygen concentration of the exhaust gas, and in the case of rich exhaust gas it is proportional to the concentration of oxygen-consuming gas molecules. Thus, it is possible to ascertain from the pump current the oxygen partial pressure of the exhaust gas, or the partial pressure of the gas molecules consuming oxygen.
From German Published Patent Application No. 199 60 329, a gas sensor having a similar sensor element is known. In contrast to the sensor element described in German Published Patent Application No. 199 41 051, the measuring-gas region and the reference-gas region are arranged in different planes of stratification. The surfaces of the outer pump electrode and the inner pump electrode are identical.
It is disadvantageous in such sensor elements that an overswinger or a counterswing is generated in the sensor signal in response to a change in the direction of the pump current, which occurs during operation of the gas sensor in a change from lean to rich exhaust gas, for example. This so-called λ=1 ripple has a detrimental effect on the evaluation of the sensor signal.